Stack molding advantageously permits molding machine output to be multiplied without appreciably increasing the overall size of the machine. However, stack molding has the disadvantage that a more extensive melt runner system is required to extend through the moving platens to reach the cavities.
It is well known that the configuration of a melt distribution path through an injection stack mold critically affects the overall part quality. Failure to provide a melt flow under equal pressure to each mold cavity will result in differential filling of the cavities and will not produce consistent parts from cavity to cavity. Typically, even pressures from cavity to cavity are ensured by providing equal length runner passages with an identical number of bends of identical radius and arc. This is usually achieved by locating the main distribution manifold centrally within the stack mold, usually within one of the moving platens.
To transfer pressurized melt to the moving platen (ie. across the first parting line between the stationary platen and the moving platen), typical applications have provided a sprue bar extending through the stationary platen from the machine nozzle across to the moving platen, as shown in U.S. Pat. No. 5,011,646 to Berteschi. This structure has the disadvantages that the sprue bar is in the way when the mold is open and damages falling parts. Furthermore, the sprue bar interferes with any robotic arm which may be provided to assist with part ejection, mold face preparation or the like.
The extensive runner system makes the use of a sprue bar system even more unsatisfactory in multi-level stack molds. For example, U.S. Pat. No. 5,370,523 to Kushnir and European Patent Application No. EP-911139 disclose a centrally located sprue bar arrangement for feeding pressurized melt to the various moving platens of a multi-level stack mold. The presence of the central sprue bar, however, limits the ability of mold larger parts, due to the interference of the sprue bar location and the mold cavity placement.
U.S. Pat. No. 5,846,472 to Rozema et al. teaches a more complex eccentric sprue bar arrangement for use in three- and four-level stack molds. The numerous sprue bars, however, only compound the problems noted above. Furthermore, the presence of multiple sprue bars can limit the size of parts that can be molded.
Another problem associated with multi-level stack molds is that the maximum height of parts to be molded is limited by the distance that the molding machine can move between its open and dosed positions and the amount of space required for each mold level. For example, the telescoping configuration of the sprue bars of EP-911139 must be made more extensive if wider platen separation is desired. Rozema et al. teach providing a bifurcated sprue bar to permit greater separation of platens upon mold parting, however, the limitations of Rozema et al. have been noted above.
Accordingly, there is a need for a melt distribution arrangement for multi-level stack molds which has generally equal length melt paths for each mold level. Furthermore, there is a need for a melt distribution arrangement for a multi-level stack mold which does not require a centrally-located sprue bar, thereby allowing single parts to be molded which extend across the central mold axis. There is yet a further need for a melt distribution arrangement for a multi-level stack mold which utilizes a minimal number of sprue bars to minimize interference with the molding process. There is also a need for an improved drool control apparatus for use in multi-level stack molds.